1. Field of the Invention
The present invention relates to optical pickup devices, and more particularly, to an optical pickup device suitably used to irradiate laser light on a disk in which a plurality of recording layers are laminated.
2. Description of the Related Art
An optical pickup device for focusing a laser beam onto a disk recording surface is arranged in an optical disk drive which records and reproduces information in and from an optical disk such as a CD (Compact Disc) and a DVD (Digital Versatile Disc).
FIG. 17 shows a basic configuration of the optical pickup device. In FIG. 17, the numeral 11 designates a semiconductor laser, the numeral 12 designates a diffraction grating, the numeral 13 designates a beam splitter, the numeral 14 designates a collimator lens, the numeral 15 designates an objective lens, the numeral 16 designates a cylindrical lens, and the numeral 17 designates a photodetector.
The laser beam emitted from the semiconductor laser 11 is divided into a main beam (0-order diffraction light) and two sub-beams (±1-order diffraction light) by the diffraction grating 12, and the light beams are incident on the beam splitter 13. The laser beams transmitted through the beam splitter 13 are converted into substantially parallel light by the collimator lens 14, and the laser beams are focused on the disk recording surface by the objective lens 15.
The light reflected from the disk reversely proceeds the optical path in which the light is incident on the disk, and the light is partially reflected by the beam splitter 13. After astigmatism is introduced by the cylindrical lens 16, the light is focused on a light receiving surface of the photodetector 17. In the configuration shown in FIG. 7, an astigmatism method is adopted as a technique of detecting focus error.
FIG. 8A shows an arrangement of spots of the three beams (main beam and sub-beams) on the disk recording surface. FIG. 8A shows the state where the three beams are focused on the disk on which grooves and lands are arranged.
As shown in FIG. 8A, in the recording and reproducing operation, the main beam is focused on a groove and the two sub-beams are separately focused on lands which sandwich the groove from both sides. The spots of FIG. 8A are arranged to perform good tracking error detection by a differential push-pull method to be described later.
FIG. 8B shows light intensity distribution of the main beam and two sub-beams on the disk recording surface.
The recording in the disk is performed only by the main beam and the two sub-beams are used to generate a tracking error signal and a focus error signal. Light intensity of the main beam is set much higher than light intensity of the sub-beams. This is because a laser output from the semiconductor laser 11 is efficiently utilized in the recording. A recording speed to the disk can be higher as the laser beam intensity is increased on the recording surface. Therefore, the laser output from the semiconductor laser 11 is divided into the main beam and the sub-beams such that an intensity portion of the main beam used in the recording is much higher than those of the sub-beams.
A light intensity ratio between the main beam and the sub-beam is determined by diffraction efficiency (usually grating depth) of the diffraction grating 12. Usually the main beam intensity is 10 to 18 times the sub-beam intensity. The ratio is directly reflected on an intensity ratio between the main beam and the sub-beam on the light receiving surface of the photodetector 17.
FIG. 9A illustrates a principle of tracking error detection by the differential push-pull method.
Referring to FIG. 9A, the numerals 171, 172, and 173 designate a quadrant sensor arranged on the photodetector 17. The main beam is accepted by the quadrant sensor 171, and the two sub-beams accepted by the quadrant sensors 172 and 173 respectively. FIG. 9A shows focusing spots of the main beam and the sub-beam located on the quadrant sensors 171, 172, and 173. Light intensity distribution is schematically shown in each spot, and hatching is performed such that the color is brought close to black as the light intensity is increased.
As shown in FIG. 9A, the letters A to L designate sensor units of the quadrant sensors 171, 172, and 173 respectively. Assuming that PA to PL are detection outputs of the sensor units A to L, a differential push-pull signal (DPP) is given by the following equation.DPP={(PA+PB)−(PC+PD)}−k1·{(PE+PF+PI+PJ)−(PG+PH+PK+PL)}  (1)
At this point, the coefficient k1 corresponds to a sensitivity multiplying factor of a sub-light receiving unit, and the coefficient k1 is set such that the detection output of the main beam is equal to the summation of the detection outputs of the sub-beams.
As shown in FIG. 8A, when the main beam is in the state where the main beam is focused at the center position of the track (groove), the main beam and two sub-beams located on the light receiving surface of the photodetector 17 become the spot states shown in part (a-2) of FIG. 9A. In this case, the light intensity distribution of each spot becomes symmetry in relation to one parting line of the quadrant sensor. Accordingly, when the computation is performed by the equation (1), the differential push-pull signal (DPP) becomes zero.
When the main beam is displaced in the radial direction (vertical direction in the paper plane) from the state shown in FIG. 8A, the main beam and two sub-beams located on the light receiving surface of the photodetector 17 become the spot states shown in part (a-1) or (a-3) of FIG. 9A. Parts (a-1) and (a-3) of FIG. 9A shows the states in which the main beam generates track shift from the center of the track toward an outer circumference direction and an inner circumference direction of the disk respectively.
In this case, the light intensity distribution of the main beam and two sub-beams located on the light receiving surface become the state in which the light intensity distribution is biased in the horizontal direction of the paper plane. As can be seen from comparison of parts (a-1) and (a-3) of FIG. 9A, the bias direction of the light intensity distribution in each spot becomes opposite according to the track shift direction of the main beam. The main beam differs from the sub-beam in that the direction in which the light intensity is biased is opposite.
The reason why the direction in which the light intensity is biased is not orthogonal to the direction in which the three spots are arranged (track direction) is that the intensity distribution within the spot is transformed by 90 degrees by the astigmatic action.
When the computation is performed by the equation (1), the differential push-pull signal (DPP) becomes a negative value in the state shown in part (a-1) of FIG. 9A, and becomes a positive value in the state shown in part (a-3). Accordingly, the track shift of the main beam on the disk can be detected based on the differential push-pull signal (DPP).
In a so-called one-beam push-pull method, a push-pull signal is generated only from the main beam, and the track shift of the main beam is detected based on the push-pull signal. However, in the one-beam push-pull method, a DC offset is generated in the push-pull signal due to inclination of the disk and an optical axis shift of the objective lens, which results in degradation of accuracy of track shift detection. On the other hand, in the differential push-pull method, the DC offset is cancelled by the computation of the equation (1), so that the accuracy of track shift detection can be enhanced.
FIG. 9B illustrates a principle of focus error detection by the differential astigmatism method. In this case, the focusing spots of the main beam and two sub-beams located on the light receiving surface of the photodetector 17 are changed from a perfect circle to an ellipse according to a focus shift.
When the main beam is focused on the disk recording surface, the spot shapes of the main beam and two sub-beams located on the light receiving surface of the photodetector 17 become substantially a perfect circle as shown in part (b-2) of FIG. 9B. On the other hand, when the focal position of the main beam is shifted forward and backward with respect to the disk recording surface, the spot shapes of the main beam and two sub-beams located on the light receiving surface of the photodetector 17 are deformed as shown in part (b-1) or (b-3) of FIG. 9B.
In this case, a differential astigmatism signal (DAS) is obtained by the following equation.DAS={(PA+PC)−(PB+PD)}−k2·{(PE+PG+PI+PK)−(PF+PH+PJ+PL)}  (2)
where k2 is a coefficient which has the same meaning as k1.
In the on-focus state shown in part (b-2) of FIG. 9B, because the main beam and two sub-beams located on the light receiving surface of the photodetector 17 have the spot shape of substantially perfect circle, when the computation of the equation (2) is performed, the differential astigmatism signal (DAS) becomes zero. On the contrary, when the focal position of the main beam is shifted forward and backward from the recording surface, the spot shape of each beam is deformed into an ellipse in a different direction depending on the focus shift direction as shown in parts (b-1) and (b-3) of FIG. 9B. Therefore, when the computation of the equation (2) is performed, the differential astigmatism signal (DAS) becomes sometimes negative ((b-1) of FIG. 9B), and sometimes positive ((b-3) of FIG. 9B). Accordingly, the focus shift of the main beam on the disk recording surface can be detected based on the differential astigmatism signal (DAS).
As with the track shift detection, in the focus shift detection, the focus error signal can be generated only from the main beam. However, when the focus error signal is generated only from the main beam, the push-pull signal is superposed as a noise on the focus error signal in traversing the track of the spot on the disk, which results in a problem that a good focus error signal cannot be obtained. On the contrary, in the differential astigmatism method, because the push-pull signal which is a noise is cancelled by the computation of the equation (2), the good focus error signal can be obtained.
Thus, in order to enhance the accuracy of tracking error signal and focus error signal, the detection signal based on the sub-beam plays a significant role.
A disk (hereinafter referred to as “multi-layer disk”) in which a plurality of recording layers are laminated has been developed and commercialized in response to a demand of recording large-capacity information in the disk. In the next-generation DVD which is currently being commercialized, the recording layers can be laminated corresponding to a blue laser beam having a wavelength of about 400 nm.
The differential push-pull method and the differential astigmatism method can be adopted even in this kind of multi-layer disks. However, when these techniques are used on the multi-layer disk, the light (stray light) reflected from the recording layer except the recording layer of the recording and reproducing target is incident on the photodetector 17, which results in a problem of lowering the accuracy of focus error signal and tracking error signal. This is so-called a problem of signal degradation caused by the stray light.
FIGS. 10A and 10B show a stray light generation state where a laser beam is focused on a multi-layer disk having two recording layers. In FIGS. 10A and 10B, the signal light (light reflected from the recording layer which is of the recording and reproducing target) is shown with a solid line, and the stray light is shown with a broken line.
FIG. 10A shows a state in which the laser beam emitted from the optical pickup device is focused on a recording layer L1. In this case, the light which is transmitted through the recording layer L1 and reflected from a recording layer L0 becomes the stray light. Because the light reflected from the recording layer L0 becomes divergent light whose starting point is located farther than the recording layer L1 with respect to the objective lens 15, the light becomes a slightly focused state compared with the parallel light, after transmitted through the objective lens 15. Accordingly, because the focal point by the collimator lens 14 is brought close to the disk side of the light receiving surface of the photodetector 17, the spot becomes a widely spread spot on the light receiving surface of the photodetector 17.
FIG. 10B shows a state in which the laser beam emitted from the optical pickup device is focused on the recording layer L0. In this case, the light reflected from the recording layer L1 becomes the stray light. Because the light reflected from the recording layer L1 becomes divergent light whose starting point is located closer to the objective lens 15 compared with the recording layer L0, the light becomes a slightly divergent state compared with the parallel light, after transmitted through the objective lens 15. Accordingly, because the focal point by the collimator lens 14 is separated from the disk with respect to the light receiving surface of the photodetector 17, the spot becomes a widely spread spot on the light receiving surface of the photodetector 17.
FIG. 11 shows an irradiation state of the stray light on the light receiving surface of the photodetector 17. In this case, the light receiving surface is irradiated with the stray light such that all the quadrant sensors 171, 172, and 173 are covered with the stray light. There are three stray light beams including the stray light based on the main beam and the stray light based on the two sub-beams, the stray light of the sub-beam is also incident on the light receiving surface while overlapping the stray light of the main beam. However, the stray light of the sub-beam has light intensity which has little influence on the focus error signal and tracking error signal, so that only the stray light of the main beam is shown in FIG. 11 for convenience sake.
FIG. 12 shows light intensity distribution of the signal light and the stray light on the light receiving surface of the photodetector 17. As will be described later, the stray light constitutes an interference fringe on the light receiving surface of the photodetector 17 by phase modulation action of the diffraction grating 12. Therefore, in the intensity distribution of actual stray light, a strong portion and a weak portion are repeated in the form of a wave corresponding to the interference fringe. The stray light intensity distribution of FIG. 12 expresses an envelope curve of the change in intensity caused by the interference fringe.
As shown in FIG. 12, peak intensity of the stray light is considerably lower than peak intensity of the signal light of the main beam. Therefore, the stray light has little influence on the signal light of the main beam. On the other hand, because the stray light intensity at the position of the sub-beam is close to the intensity of the signal light of the sub-beam, sometimes a behavior of the stray light has a strong influence on the sub-beam detection signal.
FIG. 13 schematically shows the interference fringe generated by the stray light on the light receiving surface. As shown in FIG. 13, the spacing between the interference fringes is usually such that several interference fringes are cast on a set of quadrant sensors. When incident positions of the interference fringes are fixed with respect to the quadrant sensors, the influence of the interference fringe on the sub-beam detection signal can be removed by appropriately performing signal processing. However, as is clear from the comparison of the regions S1 in FIGS. 13, 14A, and 14B, the incident positions of the interference fringes with respect to the quadrant sensors change over time in actuality. This may be attributed to inclination of the optical disk, a temporal change in optical path length, an uneven thickness of the optical disk substrate, uneven birefringent distribution, and the like.
In such cases, assuming that the light intensity does not change along the interference fringe line in one interference fringe, the influences caused by the change in position of the interference fringes cancel each other by the computation of the equation (2) with respect to the above-described focal error signal (differential astigmatic signal: DAS). However, for the tracking error signal (differential push-pull signal: DPP), the influences do not cancel each other even if the equation (1) is computed.
An influence of the change in position of the interference fringe on the tracking error signal will be described with reference to FIGS. 15A to 15C. FIGS. 15A to 15C show examples of the change in interference fringe near the region S1 of FIGS. 13, 14A, and 14B. On the optical disk, it is assumed that the main spot is located at the center of the track while the signal spot on the photodetector 17 is in the state shown in (a-2) of FIG. 9A.
In this case, because the light intensity distribution in the main beam and sub-beams are symmetrical relative to one sensor parting line (line dividing the quadrant sensor 172 into sensor units E and F and sensor units H and G), the tracking error signal DPP becomes zero by the computation of the equation (1) unless the stray light has any influence. However, in consideration of the influence of the stray light, when the stray light interference fringe is located at the position of FIG. 15 with respect to the light receiving surface of the photodetector 17, the stray light intensity distribution becomes asymmetrical relative to the sensor parting line, so that the computation result of the equation (1) takes a value except zero.
When the stray light interference fringe is changed from the state of FIG. 15A to the state of FIG. 15B, the light quantity corresponding to an area IEF1 of FIG. 15B is increased in regions (E+F) of the sensor units E and F. On the other hand, in regions (G+H) of the sensor units G and H, the light quantity corresponding to an area IGH1 of FIG. 15B is increased while the light quantity corresponding to an area DGH1 is decreased. However, because the area of DGH1 of FIG. 15A is larger than the area of IGH1, in the regions (G+H) of the sensor units G and H, the total amount of light received is decreased as compared with the case of FIG. 15A. Accordingly, the computation result (tracking error signal DPP) by the equation (1) in the state of FIG. 15B is further changed from the case of FIG. 15A.
When the stray light interference fringe is changed from the state of FIG. 15B to the state of FIG. 15C, in the regions (E+F) of the sensor units E and F, the light quantity corresponding to an area IEF2 of FIG. 15C is increased while the light quantity corresponding to an area DEF2 is decreased. Because the area of IEF2 is larger than the area of DEF2 of FIG. 15B, the total amount of light received is increased as compared with the case of FIG. 15B. On the other hand, in the regions (G+H) of the sensor units G and H, the light quantity corresponding to an area IGH2 of FIG. 15C is increased while the light quantity corresponding to an area DGH2 is decreased. However, because the area of DGH2 of FIG. 15B is substantially equal to the area of IGH2 of FIG. 15C, the total amount of light received is substantially unchanged as compared with the case of FIG. 15B. Accordingly, the computation result (tracking error signal DPP) by the equation (1) in the state of FIG. 15C is further changed from the case of FIG. 15B.
Thus, the magnitude of the tracking error signal is sequentially changed according to the temporal change in position of the stray light interference fringe. At this point, because the position of the stray light interference fringe is usually changed at substantially the same frequency as the tracking error signal, the influence of the stray light interference fringe on the tracking error signal is generated in the form of a strong noise.
Therefore, the following techniques are proposed to solve the problem. FIG. 16A shows a configuration of the optical pickup device according to a first technique. In the configuration example of FIG. 16A, a light shielding member is inserted into an optical path of the laser beam, and the stray light is blocked by a light shielding portion provided on the light shielding member. At this point, FIG. 16B shows the spot states of the main beam and sub-beam and the stray light irradiation state on the light receiving surface of the photodetector.
As shown in FIG. 16B, in the configuration example, the stray light is prevented from entering the quadrant sensor. However, at the same time, because part of the signal light is also blocked by the light shielding portion, a region (shown by “N” in FIG. 16B) where the reflected light is lost is generated in the spots of the main beam and of the sub-beams on the light receiving surface of the photodetector. Particularly, the lost region in the spot of the main beam signal light becomes a problem. That is, the lost region in the spot of the main beam signal light is generated in the central portion of the spot having strong light intensity, which results in a problem of remarkably lowering quality of the RF signal or the focus error signal.
FIG. 17A shows a configuration of the optical pickup device according to a second technique. In the configuration example of FIG. 17A, a prism having two critical angle planes (first critical angle plane and second critical angle plane) is arranged between the collimator lens and the objective lens. At this point, the first critical angle plane and the second critical angle plane reflect only the light having a predetermined incident angle (critical angle) or larger. Therefore, half of the stray light is blocked in the first critical angle plane, and the other half is blocked in the second critical angle plane.
In this case, because the critical angle condition is steep, the stray light is substantially eliminated on the light receiving surface of the photodetector as shown in FIG. 17B. However, at the same time, because the sub-beam signal light is incident on the prism while shifted from the parallel light state, the sub-beam signal light is also blocked when it is incident on the first critical angle plane and the second critical angle plane, and is not introduced onto the light receiving surface of the photodetector as shown in FIG. 17B.